In wound rotor induction generators, the currents in the secondary winding (usually the rotor) are created solely by induction. These currents result from voltages induced in the secondary windings by the rotating magnetic fields in the primary winding arising from the application of line voltages thereto. The rotor windings are generally connected to slip rings. When the rotor is driven at above the synchronous speed, either shorted or with proper equipment attached thereto as with a load connected to the slip rings, the machine acts as a generator over a wide range.
In general an induction generator with a squirrel-cage rotor has a "closed door" effect when overloaded. At overload, which occurs when the speed exceeds the synchronous speed by an amount slightly above the motor slip speed, the generator stops delivering power and offers little resistance to the prime mover. This occurs at rotary speeds quite close to normal operation. When the load is lost, the prime mover and the induction generator race to destruction together.
For the past thirty years or more when driving a generator or a squirrel-cage type induction generator with various types of prime movers, the speed of the electrical generator was kept nearly constant by employing mechanical methods for controlling the speed of the prime mover. These mechanical methods also limited the speed of the prime mover when the load was lost.
For generators driven by wind power, spoilers or blade pitch control or mechanical brakes are used to limit the speed. These methods have a very short life and are costly to maintain.
The frequency in the wound secondary of the generator is in direct proportion to the rpm above the base speed. Thus, in a 900 rpm, 8 pole, 60 Hz machine, the frequency in the secondary at 900 rpm is zero. The rise in frequency is 60 Hz for each 900 rpm above base speed. So it is 30 Hz (Hertz, cycles) at 1350 rpm (900+450=1350) and 60 Hz (cycles) in the secondary of this machine at 1800 rpm. This is important; it means the generator is being driven in the same direction that it would run as a motor. Thus, there is no need to reverse any connections to use the generator as a motor for assisting the generator to reach base speed by means of any variable speed prime mover or power source, including wind and wave power.